1. Field of the Invention
The present invention relates to an improved urea synthesis process, and more particularly to a urea synthesis process wherein heat is recovered and utilized for decomposing unreacted ammonium carbamate contained in a urea solution.
2. Description of the Related Art
With regard to a method for recovering heat in a urea synthesis process, the present applicant has already proposed some methods in e.g. Japanese Patent Examined Publication No. 62-15070, Japanese Patent Laid-Open Nos. 61-109760 and 10-182587.
In the method disclosed in the above listed Japanese Patent Examined Publication No. 62-15070 or Japanese Patent Laid-Open No. 61-109760, as shown in FIG. 3, a mixed gas separated from a urea synthesis solution in a stripper 2 is divided and supplied separately to the tube side of each of two shell-and-tube type vertical condensers 9a and 9b which are located in parallel, throughlines 18, and 32 and 33 so that the mixed gas is condensed. In the vertical condenser 9a, water is supplied to a shell side thereof through a line 36 and low pressure steam is generated through a line 37 by heat generated by condensation of the mixed gas. In the vertical condenser 9b, aurea solution comprising unseparated unreacted ammonia and unreacted carbon dioxide fed from the stripper 2 is heated at a shell side thereof by heat generated by condensation of the mixed gas, so that the heat is used as a heat source in a step of separating unreacted ammonia and unreacted carbon dioxide in a urea synthesis process.
According to this method, the condensation temperature of the mixed gas comprising ammonia, carbon dioxide and water, which is brought from the stripper 2, is 170xc2x0 C. to 180xc2x0 C. Since this temperature is higher than the temperature of the low pressure steam, which is 150xc2x0 C. to 155xc2x0 C., the temperature of a heated side can be kept higher and thereby heat recovery can efficiently be carried out. In addition, it is also possible to install a medium pressure decomposer, reboiler or falling film heater (not shown) in stead of the vertical condenser 9b to recover heat in the same manner.
However, in the case of the above described heat recovery, because of the increased number of devices used for urea production, the necessity of distribution of a mixed gas to two vertical condensers, etc., a urea synthesis system becomes complicated. Moreover, when condensation of the mixed gas is carried out on the tube side of the vertical condenser, due to thin-film condensation occurred on the tube surface, the heat transfer performance becomes lower and the heat transfer area becomes larger when compared with the bubble column type vertical condensation reactor described in Japanese Patent Laid-Open No. 10-182587. This results in upsizing of a vertical condenser, a high production cost of devices, and by extension, expensive construction cost. Furthermore, by this method, the mixed gas is supplied to the tube side of each of the vertical condensers 9a and 9b whereas a cooling medium is supplied to the shell side, and therefore this method is not structurally suitable for dividing the shell side, supplying a plurality of cooling mediums and performing heat exchange with each of them.
In the above Japanese Patent Laid-Open No. 10-182587, as shown in FIG. 4, the mixed gas discharged from the stripper 2 to the line 18 is supplied to a vertical condensation reactor 4 having a cooling tube 23 therein and is then condensed, and the obtained heat of condensation is recovered by generating a low pressure steam in the cooling tube 23. The generated low pressure steam is used in a step of separating unreacted ammonia and carbon dioxide under a low pressure or in a step of concentrating a urea solution in a urea synthesis process.
However, when the recovered low pressure steam having a low temperature is used, it becomes difficult to maintain a temperature difference for heating in a step of separating unreacted ammonia and carbon dioxide, which requires a high temperature, and therefore it is required to enlarge the heat transfer area of a heater of a separator.
As described above, the prior art regarding a heat recovery method in a urea synthesis process has various problems to be solved.
It is an object of the present invention to provide a method for effectively recovering heat in a urea synthesis process so as to sharply reduce a heat transfer area, which is required in a urea production process as a whole.
As a result of concentrated studies to solve the above mentioned problems of the prior art, the present inventors have found that, when the heat of condensation of a mixed gas of ammonia and carbon dioxide having a high temperature, that is, a high temperature of 170xc2x0 C. to 180xc2x0 C., is directly used as a heat source in a step of separating unreacted ammonia and carbon dioxide, without generating a low pressure steam, in other words, without reducing the temperature to 150xc2x0 C. to 155xc2x0 C. that is the temperature of the low pressure steam, heat transfer efficiency is substantially improved and the heat transfer area required in a urea synthesis process as a whole is sharply reduced, thereby completing the present invention.
The present invention provides the following urea synthesis process:
A urea synthesis process, which comprises the steps of, reacting raw material ammonia and carbon dioxide in a urea synthesis column at a urea synthesis temperature and under a urea synthesis pressure; contacting the resulting urea synthesis solution comprising urea, unreacted ammonia, unreacted carbon dioxide and water with at least a portion of the raw material carbon dioxide under heating in a stripper under a pressure approximately equal to the urea synthesis pressure, thereby separating unreacted ammonia and unreacted carbon dioxide as a mixed gas of ammonia, carbon dioxide and water; feeding the mixed gas to a shell side of a vertical condensation reactor having a cooling tube therein to contact the mixed gas with an absorption medium under cooling for condensation of the mixed gas; and recycling the resulting condensate to the urea synthesis column so as to obtain a urea synthesis solution comprising unreacted ammonia and unreacted carbon dioxide that have not been separated, wherein the urea synthesis solution separated from the mixed gas in the stripper is fed to the cooling tube of the vertical condensation reactor, while supplying the mixed gas separated from the urea synthesis solution in the stripper is fed to the shell side of the vertical condensation reactor so as to condense the mixed gas by indirect heat exchange with the urea synthesis solution in the cooling tube; and the urea synthesis solution is heated in the cooling tube by the heat of condensation generated at the time of the condensation of the mixed gas.
It is preferable that the urea synthesis solution comprising unseparated unreacted ammonia and unreacted carbon dioxide discharged from the stripper is decompressed to a pressure lower than the urea synthesis pressure, and is then fed to the cooling tube of the vertical condensation reactor.
It is preferable that the urea synthesis solution comprising unseparated unreacted ammonia and unreacted carbon dioxide discharged from the stripper is decompressed and fed to a medium pressure separator wherein a portion of unreacted ammonia and unreacted carbon dioxide contained in the urea synthesis solution is separated, and then the resulting solution is fed to the cooling tube of the vertical condensation reactor.
The urea synthesis solution fed to the cooling tube side of the vertical condensation reactor is preferably heated to a temperature of 150xc2x0 C. to 170xc2x0 C. by the heat of condensation of the mixed gas fed to the vertical condensation reactor.
The cooling tube of the vertical condensation reactor is preferably a U-shaped tube having multi channels.